10.3
Cis-regulatory Sequences
Cis-regulatory sequences are short fragments of non-coding DNA that are present on the same chromosomes as the genes that they regulate. These fragmen…
Transcriptional regulators are proteins that bind to short, specific stretches of non-coding DNA, known as cis-regulatory sequences. These essential components for gene regulation are present throughout the genome.
Transcription regulators can bind to cis-regulatory sequences on DNA in its double helix form. The helical form of DNA has two grooves that differ in size. The wider groove, known as the major groove, is more accessible than the smaller minor groove, resulting in most of the interactions occurring at the major groove.
The edges of the nitrogenous bases are exposed along the grooves and are capable of hydrogen bonding and other noncovalent interactions.
This binding of transcriptional regulators is sequence-specific; however, closely related sequences are also recognized. This association is highly stable due to the multiple non-covalent interactions involved.
Cis-regulatory sequences can govern every aspect of gene transcription, including the inhibition or initiation of transcription, as well as the rate of transcription.
They can be broadly classified into those that are in close proximity to the site of transcription and those that are thousands of base pairs away. The ones that are close are usually part of the promoter sequences that initiate gene transcription.
Those that are farther away can participate in either enhancing or silencing the transcription. Often the compact packing of the DNA in the chromosome causes these sequences to be spatially close to the gene that they regulate even if they are far away in the sequence.
A combination of several cis-regulatory sequences controls the expression of most genes. Rarely, a gene will be under the control of only a single sequence.
View the full transcript and gain access to JoVE Core videos
Q1: What are cis-regulatory sequences and where are they found in the genome?
Cis-regulatory sequences are short, specific stretches of non-coding DNA that bind transcriptional regulators to control gene expression. These sequences are present throughout the genome on the same chromosomes as the genes they regulate. They can be located close to genes or thousands of base pairs away, yet DNA's compact packaging in chromosomes often positions distant sequences spatially near their target genes.
Q2: How do transcriptional regulators recognize and bind to cis-regulatory sequences?
Transcriptional regulators recognize cis-regulatory sequences through sequence-specific binding to DNA's double helix form. The nitrogenous base edges exposed along DNA grooves display characteristic chemical features, including hydrogen bond donors and acceptors. Regulators preferentially bind the major groove due to its larger size, forming multiple stable non-covalent interactions with the bases.
Q3: What functions do cis-regulatory sequences perform in gene expression?
Cis-regulatory sequences govern every aspect of gene transcription, including initiation, inhibition, and transcription rate control. Close sequences typically function as promoter regions that initiate transcription, while distant sequences enhance or silence transcription. Most genes are controlled by multiple cis-regulatory sequences working together, though rarely a single sequence regulates a gene.
Q4: Why is the major groove of DNA more important for transcription factor binding than the minor groove?
The major groove is wider and more accessible than the minor groove, making it the preferred binding site for transcriptional regulators. The larger groove exposes nitrogenous base edges more effectively, allowing regulatory proteins to recognize sequence-specific chemical features and form stable interactions. This accessibility difference explains why most transcriptional regulator-DNA interactions occur at the major groove.
Q5: How does DNA packaging affect the regulation of distant cis-regulatory sequences?
Although some cis-regulatory sequences are thousands of base pairs away from their target genes in linear DNA sequence, the compact packing of DNA within chromosomes brings these distant sequences into spatial proximity with regulated genes. This three-dimensional organization allows transcriptional regulators bound to far-away sequences to influence gene transcription effectively, enabling complex regulatory control despite linear distance.
Q6: Are transcriptional regulators strictly specific to single DNA sequences?
Transcriptional regulators exhibit sequence-specific binding, recognizing particular cis-regulatory sequences with high precision. However, they also recognize closely related sequences, demonstrating some binding flexibility. This specificity combined with tolerance for similar sequences allows regulatory proteins to fine-tune gene expression while maintaining control over their primary target sequences.
Q7: What types of molecular interactions stabilize transcriptional regulator binding to cis-regulatory sequences?
Transcriptional regulator binding involves multiple non-covalent interactions, including hydrogen bonds between base edges and regulatory proteins. These weak individual interactions collectively create highly stable associations between regulators and cis-regulatory sequences. The cumulative effect of many hydrogen bonds and other non-covalent forces ensures strong, specific binding necessary for reliable gene regulation.